1. Field of the Invention
The present invention relates generally to amplifiers, and, more particularly to a technique to provide a controlled bias setting for a FET common source RF amplifier that can operate with a large signal present.
2. Description of the Related Art
FIG. 1 illustrates a schematic of a conventional Class A RF Power Amplifier with bias control used to amplify an RF signal. As shown, the conventional Class A RF Power Amplifier 10 includes an RF input 20 and an RF output 40, which outputs an amplified signal of the signal applied to RF input 20. Furthermore, a first DC blocking capacitor 22 is coupled between RF input 20 and an input matching circuit 24, which typically consists of an LC circuit. Likewise, a second DC blocking capacitor 42 is coupled between RF output 40 and an output matching circuit 44, which also typically consists of an LC circuit. The Class A RF Power Amplifier 10 also includes a switching transistor 30, which is always biased “ON” so that it conducts during one complete cycle of the input signal producing minimum distortion and maximum amplitude to the output.
Moreover, gate voltage bias 32 and input bias tee 34 are coupled to the gate of the switching transistor 30 and a drain voltage bias 36 and output bias tee 38 are coupled to the drain of the switching transistor 30. In this example of FIG. 1, input bias tee 34 and output bias tee 38 each comprise a simple inductor, although it should be appreciated that a capacitor can be provided across each of the gate voltage bias 32 and drain voltage bias 36 for non-ideal cases as would be understood to one skilled in the art. Conceptually, it should be appreciated that the inductors block the AC bias, but allows the DC bias from the gate voltage bias 32 and drain voltage bias 36, respectively.
For semiconductor devices used in power applications, such as the switching transistor 30 of amplifier 10 shown in FIG. 1, gallium nitride (GaN) semiconductor devices are increasingly desirable because of their ability to carry large current and support high voltages. Development of these devices has generally been aimed at high power/high frequency applications. Devices fabricated for these types of applications are based on general device structures that exhibit high electron mobility and are referred to variously as heterojunction field effect transistors (HFET), high electron mobility transistors (HEMT), or modulation doped field effect transistors (MODFET).
A GaN HEMT device includes a nitride semiconductor with at least two nitride layers. Different materials formed on the semiconductor or on a buffer layer cause the layers to have different band gaps. The different material in the adjacent nitride layers also causes polarization, which contributes to a conductive two dimensional electron gas (2DEG) region near the junction of the two layers, specifically in the layer with the narrower band gap. If the 2DEG region is depleted, i.e. removed, below the gate at zero applied gate bias, the device can be an enhancement mode device. Enhancement mode devices are normally off and are desirable because of the added safety they provide and because they are easier to control with simple, low cost drive circuits. An enhancement mode device requires a positive bias applied at the gate in order to conduct current.
FIG. 2 illustrates the transfer characteristics of an enhancement mode GaN® FET, i.e., a eGAN FET. As shown, an input voltage is applied to the eGaN FET and a bias point is determined. The bias point and the characteristics of the FET are used to amplify the input signal by effectively shifting the current based on the input voltage swing.
It is generally understood that the simplest form of MOSFET bias is to apply a fixed voltage to the gate which, in turn, defines the drain current (assuming linear region operation). This approach works well if there are no temperature variation effects. This is also the approach commonly used to control bias of a Class A Power RF amplifier 10, as shown above in FIG. 1.
Another conventional method to control bias is monitoring drain current feedback. This approach can be used for small signal amplification. The primary drawback of this approach is that when the RF power is high, it contributes significantly to the drain current and the bias controller will throttle back the bias setting in response. This transitions the amplifier from a class A to a class AB amplifier or even further to a class B amplifier or class C amplifier in extreme cases.
A third conventional method to control bias employs sample and hold circuits and is described in U.S. Pat. No. 5,488,331. In the described method, the bias control circuit employs a bias current sampling circuit for each associated amplifying device that measures the output current of its associated amplifying device and produces an output level that represents the output current of the device. Further, a sample/hold circuit samples the output level of the sampling at a time when a zero or null input level appears at the control electrode of the amplifying device. The circuit then applies a resulting output current bias signal to an input of a digital control circuit. The primary disadvantage of the approach described in U.S. Pat. No. 5,488,331 is that it is highly complex to implement and limited in its frequency use.
A fourth conventional method pulses a class A amplifier to control bias. This method adds complications as the RF choke can act as a boost inductor thereby establishing a voltage on the drain that can exceed the rating when the pulse transients occur rapidly. The challenges to large signal pulsed amplifier measurements are well-documented in S. J. Doo et al., “Pulsed-IV Pulsed RF Measurements Using a Large Signal Network Analyzer,” 65th ARFTG Conference Digest, June 2005.